Overall a fun game with great sound design. I enjoyed seeing the bullets bounce, that's not something I usually see in bullet hell style games. It adds another layer of complexity.
If you wanted to improve, I suggest resetting the player health between each boss encounter, or giving the player some sort of choice like 'more powerful weapon' or 'return to full health'. This could create an interesting choice for the player and add a lot of replay value. Secondly, add more juice, add in some particle fx when you land a hit or get hit, maybe some screen shake too.
If you keep working on this, let me know and I'd happily try it again!